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April 29, 2004 Glossary-3
Focal length The distance from the principal point to the focal point. In 35 mm-format
cameras, lenses with a focal length of approximately 50 mm are called
normal or standard lenses. Lenses with a focal length less than
approximately 35 mm are called wide angle lenses, and lenses with a focal
length more than approximately 85 mm are called telephoto lenses. Lenses
that allow the user to continuously vary the focal length without changing
focus are called zoom lenses.
Front-Curtain Sync The flash fires an instant after the front curtain of a focal plane shutter has
completed its travel across the film plane. The camera operates this way
with the flash sync mode at Normal Sync. See also “Rear-Curtain Sync.”
Guide number The guide number indicates the power of a flash in relation to ISO. Guide
numbers, quoted in either meters or feet, are used to calculate the f/stop for
correct exposure as follows:
f/stop = guide number / flash-to-subject distance
Using a selected aperture, calculate the required flash-to-subject distance
with the formula:
flash-to-subject distance = guide number / f-stop
Use to determine the maximum flash-to-subject distance.
Histogram The image histogram shows the range and distribution of tonal values for an
image. The tick marks at the bottom of the histogram represent
photographic stops relative to the factory-determined 18% gray level. The
range of stops is from +3 to -7. The double bar tick mark indicates the
exposure value associated with 18% gray. Tick marks to the right of the
double mark indicate one-stop increments brighter than 18% gray, and tick
marks to the left indicate one-stop increments darker than 18% gray.
ISO ISO is the international standard for representing film sensitivity. The higher
the number, the greater the sensitivity, and vice versa. For example, ISO 200
is twice as sensitive as ISO 100, and half that of ISO 400. When you change
the ISO setting on your camera, you adjust the sensitivity of the imager
equivalent to the corresponding film sensitivity.
LCD Liquid Crystal Display. Your camera has four: the Top Status LCD panel,
Back Status LCD panel, Image LCD panel, and the LCD inside the viewfinder.
Manual flash Flash output is controlled manually in manual flash mode, unlike in auto
flash mode, where flash output power varies automatically according to the
selected aperture. Some Speedlights provide selectable manual outputs
(full, 1/2, 1/4, 1/8, 1/16, etc.), while others provide full manual output
only.